Reposted from the Cliff Mass Weather Blog
January 10, 2021
The strongest atmospheric river of the season so far is imminent and will reach our coast tomorrow (Monday).
Atmospheric rivers are fascinating beings. Why does the atmosphere concentrate water vapor in fast moving narrow streams? And why do such currents provide so abundant rainfall and are so optimal for promoting floods and landslides? These questions are among those that I and others have researched and understood fairly well over the past few decades.
Below is a graph that shows the key parameter that characterizes the flows of the atmosphere Integrated water vapor transport (IVT)), which is forecast for Monday evening at 10 p.m. IVT is essentially the amount of water vapor that is moved by the atmosphere. Mathematically speaking, water vapor is constant times wind speed.
In this figure, the blue areas are places with very high IVT values (over 800 in the units shown).
Bringing a lot of water vapor into the area can add water to clouds and rainfall, especially if the air needs to rise in our terrain.
One group at the Scripps Institute of Oceanography in San Diego even has an atmospheric rating system similar to the Saffir-Simpson scale for hurricanes (Category 1-5) (see below) and based on IVT and duration. The event we are going to experience this week will reach at least a Category 3 level!
What makes this event particularly threatening is that there will be two main impulses with large amounts of water vapor. on a late Monday / Tuesday and the other Wednesday morning, as the forecasts for steam transport at these times show (see below). Each is connected to separate low pressure centers and high winds.
You notice how the values decrease when the plumes of moisture move inland? Why do you think this is?
The answer is that water vapor precipitates out as rain and is thus lost from the atmosphere. The mountains squeeze out the water vapor like you would with a sponge.
But you want to see the precipitation, I’m sure of that.
For the 72 hours that ends at 4:00 p.m. on Wednesday, there are 5-10 inches up windward of the regional terrain, with some locations near the California-Oregon border reaching over 10 inches. Oregon is in the porthole of this event. And such heavy rain will fall over regions where major fires broke out last September, which is cause for concern.
Atmospheric rivers are generally associated with warm air, and it is no different. Freezing point climbs to 5000-6000 feet on Tuesday. Not good for skiing in Snoqualmie.
Why are there atmospheric rivers at all? For us here in the Pacific Northwest, they’re associated with situations where pressures over California are higher than normal and pressures over Alaska are lower than normal. The result is a large pressure gradient between the two that creates a strong south-westerly current that draws warm, moist air into our region.
For an illustration, look at the forecast map of the elevations (such as pressures) at approximately 5000 feet for Tuesday morning. Temperatures are indicated by shadows and the winds by wind spikes. A low near Alaska and a high pressure / altitude of CA. A large change in pressure and thus strong winds (from the southwest) bring the warm, juicy air towards us.
This will not be an extreme, harmful event, but local flooding is expected, particularly in western Oregon.
/ Cliff Mass
AND on the same topic today I received this tip from Joel O’Bryan.
Embassy: The Pineapple Express is destined for the west coast of the USA in the middle of the week.
https://www.star.nesdis.noaa.gov/GOES/conus_band.php?sat=G17&band=Sandwich&length=12
Excellent visualization.
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